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Schmitt trigger
From Wikipedia, the free encyclopedia
In electronics, a Schmitt trigger is a comparator circuit with hysteresis, implemented by applying positive
feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which
converts an analog input signal to a digital output signal. The circuit is named a "trigger" because the output
retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration,
when the input is higher than a certain chosen threshold, the output is high. When the input is below a different
(lower) chosen threshold, the output is low, and when the input is between the two levels, the output retains its
value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and
can act as a bistable circuit (latch or flip-flop). There is a close relation between the two kinds of circuits: a
Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.
Schmitt trigger devices are typically used in signal conditioning applications to remove noise from signals used
in digital circuits, particularly mechanical switch bounce. They are also used in closed loop negative feedback
configurations to implement relaxation oscillators, used in function generators and switching power supplies.
Contents
1 Invention
2 Implementation
2.1 Fundamental idea
2.2 Transistor Schmitt triggers
2.2.1 Classic emitter-coupled circuit
2.2.1.1 Operation
2.2.1.2 Variations
2.2.2 Collector-base coupled circuit
2.3 Comparison between emitter- and collectorcoupled circuit
2.4 Op-amp implementations
2.4.1 Non-inverting Schmitt trigger
2.4.2 Inverting Schmitt trigger
3 Applications
3.1 Noise immunity
3.2 Use as an oscillator
4 See also
5 Notes
6 References
7 External links
The output of a Schmitt trigger (B) and a
comparator (A), when a noisy signal (U) is
applied. The green dotted lines are the
circuit's switching thresholds. The Schmitt
trigger tends to remove noise from the
signal.
Invention
The Schmitt trigger was invented by US scientist Otto H. Schmitt in 1934 while he was still a graduate
student,[1] later described in his doctoral dissertation (1937) as a "thermionic trigger."[2] It was a direct result
of Schmitt's study of the neural impulse propagation in squid nerves.[2]
Implementation
Fundamental idea
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Circuits with hysteresis are based on the fundamental
positive feedback idea: any active circuit can be made to
behave as a Schmitt trigger by applying a positive
feedback so that the loop gain is more than one. The
positive feedback is introduced by adding a part of the
output voltage to the input voltage; so, these circuits
contain an attenuator (the B box in the figure on the right)
and a summer (the circle with "+" inside) in addition to an
Schmitt trigger is a system with an avalanche-like
amplifier acting as a comparator. There are three specific
positive feedback (B < 1; B.A > 1), in which the
techniques for implementing this general idea. The first
output helps the input
two of them are dual versions (series and parallel) of the
general positive feedback system. In these configurations,
the output voltage increases the effective difference input voltage of the comparator by decreasing the
threshold or by increasing the circuit input voltage; the threshold and memory properties are incorporated in
one element. In the third technique, the threshold and memory properties are separated.
Dynamic threshold (series feedback): when the input voltage crosses the threshold in some direction the
very circuit changes its own threshold to the opposite direction. For this purpose, it subtracts a part of its
output voltage from the threshold (it is equal to adding voltage to the input voltage). Thus the output affects
the threshold and does not impact on the input voltage. These circuits are implemented by a differential
amplifier with series positive feedback where the input is connected to the inverting input and the output - to
the non-inverting input. In this arrangement, attenuation and summation are separated: a voltage divider acts
as an attenuator and the loop acts as a simple series voltage summer. Examples: the classic transistor emittercoupled Schmitt trigger, op-amp inverting Schmitt trigger, etc.
Modified input voltage (parallel feedback): when the input voltage crosses the threshold in some direction
the circuit changes the very input voltage in the same direction (now it adds a part of its output voltage
directly to the input voltage). Thus the output "helps" the input voltage and does not affect the threshold.
These circuits can be implemented by a single-ended non-inverting amplifier with parallel positive feedback
where the input and the output sources are connected through resistors to the input. The two resistors form a
weighted parallel summer incorporating both the attenuation and summation. Examples: the less familiar
collector-base coupled Schmitt trigger, op-amp non-inverting Schmitt trigger, etc.
Some circuits and elements exhibiting negative resistance can also act in a similar way: negative impedance
converters (NIC), neon lamps, tunnel diodes (e.g., a diode with an "N"-shaped current–voltage characteristic
in the first quadrant), etc. In the last case, an oscillating input will cause the diode to move from one rising leg
of the "N" to the other and back again as the input crosses the rising and falling switching thresholds.
Two different unidirectional thresholds are assigned in this case to two separate open-loop comparators
(without hysteresis) driving an RS trigger (2-input memory cell). The trigger is toggled high when the input
voltage crosses down to up the high threshold and low when the input voltage crosses up to down the low
threshold. Again, there is a positive feedback but now it is concentrated only in the memory cell. Example: 555
timer, switch debounce circuit.[3]
The symbol for Schmitt triggers in circuit diagrams is a triangle with a symbol inside representing its ideal
hysteresis curve.
Transistor Schmitt triggers
Classic emitter-coupled circuit
The original Schmitt trigger is based on the dynamic threshold idea that is implemented by a voltage divider
with a switchable upper leg (the collector resistors RC1 and RC2) and a steady lower leg (RE). Q1 acts as a
comparator with a differential input (Q1 base-emitter junction) consisting of an inverting (Q1 base) and a
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non-inverting (Q1 emitter) inputs. The input voltage is applied to the
inverting input; the output voltage of the voltage divider is applied to the
non-inverting input thus determining its threshold. The comparator output
drives the second common collector stage Q2 (an emitter follower)
through the voltage follower R1-R2. The emitter-coupled transistors Q1
and Q2 actually compose an electronic double throw switch that switches
over the upper legs of the voltage divider and changes the threshold in a
different (to the input voltage) direction.
This configuration can be considered as a differential amplifier with series
positive feedback between its non-inverting input (Q2 base) and output
(Q1 collector) that forces the transition process. There is also a smaller
negative feedback introduced by the emitter resistor RE. To make the
positive feedback dominate over the negative one and to obtain a
hysteresis, the proportion between the two collector resistors is chosen
RC1 > RC2. Thus less current flows through and less voltage drop is across
RE when Q1 is switched on than in the case when Q2 is switched on. As a
result, the circuit has two different thresholds in regard to ground (V- in
the picture).
Operation
A symbol of Schmitt trigger shown
with a non-inverting hysteresis
curve embedded in a buffer.
Schmitt triggers can also be shown
with inverting hysteresis curves and
may be followed by bubbles. The
documentation for the particular
Schmitt trigger being used must be
consulted to determine whether the
device is non-inverting (i.e., where
positive output transitions are
caused by positive-going inputs) or
inverting (i.e., where positive
output transitions are caused by
negative-going inputs).
Initial state. For NPN transistors as shown, imagine
the input voltage is below the shared emitter voltage
(high threshold for concreteness) so that Q1
base-emitter junction is backward-biased and Q1 does
not conduct. Q2 base voltage is determined by the
mentioned divider so that Q2 is conducting and the
trigger output is in the low state. The two resistors RC2
and RE form another voltage divider that determines
the high threshold. Neglecting VBE, the high threshold
value is approximately
.
Schmitt trigger implemented by two emitter-coupled
transistor stages
The output voltage is low but well above the ground. It
is approximately equal to the high threshold and may
not be low enough to be a logical zero for next digital circuits. This may require additional shifting circuit
following the trigger circuit.
Crossing up the high threshold. When the input voltage (Q1 base voltage) rises slightly above the voltage
across the emitter resistor RE (the high threshold), Q1 begins conducting. Its collector voltage goes down and
Q2 begins going cut-off, because the voltage divider now provides lower Q2 base voltage. The common
emitter voltage follows this change and goes down thus making Q1 conduct more. The current begins steering
from the right leg of the circuit to the left one. Although Q1 is more conducting, it passes less current through
RE (since RC1 > RC2); the emitter voltage continues dropping and the effective Q1 base-emitter voltage
continuously increases. This avalanche-like process continues until Q1 becomes completely turned on
(saturated) and Q2 turned off. The trigger is transitioned to the high state and the output (Q2 collector) voltage
is close to V+. Now, the two resistors RC1 and RE form a voltage divider that determines the low threshold. Its
value is approximately
.
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Crossing down the low threshold. With the trigger now in the high state, if the input voltage lowers enough
(below the low threshold), Q1 begins cutting-off. Its collector current reduces; as a result, the shared emitter
voltage lowers slightly and Q1 collector voltage rises significantly. R1-R2 voltage divider conveys this change
to Q2 base voltage and it begins conducting. The voltage across RE rises, further reducing the Q1 base-emitter
potential in the same avalanche-like manner, and Q1 ceases to conduct. Q2 becomes completely turned-on
(saturated) and the output voltage becomes low again.
Variations
Non-inverting circuit. The classic non-inverting Schmitt trigger can be
turned into an inverting trigger by taking Vout from the emitters instead of
from a Q2 collector. In this configuration, the output voltage is equal to
the dynamic threshold (the shared emitter voltage) and both the output
levels stay away from the supply rails. Another disadvantage is that the
load changes the thresholds; so, it has to be high enough. The base resistor
RB is obligatory to prevent the impact of the input voltage through Q1
base-emitter junction on the emitter voltage.
Direct-coupled circuit. To simplify the circuit, the R1–R2 voltage divider
can be omitted connecting Q1 collector directly to Q2 base. The base
resistor RB can be omitted as well so that the input voltage source drives
directly Q1 base.[4] In this case, the common emitter voltage and Q1
collector voltage are not suitable for outputs. Only Q2 collector should be
used as an output since, when the input voltage exceeds the high threshold
and Q1 saturates, its base-emitter junction is forward biased and transfers
the input voltage variations directly to the emitters. As a result, the
common emitter voltage and Q1 collector voltage follow the input voltage.
This situation is typical for over-driven transistor differential amplifiers
and ECL gates.
Collector-base coupled circuit
Like every latch, the fundamental collector-base coupled bistable circuit
possesses a hysteresis. So, it can be converted to a Schmitt trigger by
connecting an additional base resistor R to some of the inputs (Q1 base in
the figure). The two resistors R and R4 form a parallel voltage summer
(the circle in the block diagram above) that sums output (Q2 collector)
voltage and the input voltage, and drives the single-ended transistor
"comparator" Q1. When the base voltage crosses the threshold (VBE0 ∞
0.65 V) in some direction, a part of Q2 collector voltage is added in the
same direction to the input voltage. Thus the output modifies the input
voltage by means of parallel positive feedback and does not affect the
threshold (the base-emitter voltage).
Symbol depicting an inverting
Schmitt trigger by showing an
inverted hysteresis curve inside a
buffer. Other symbols show a
hysteresis curve (which may be
inverting or non-inverting)
embedded in a buffer followed by a
bubble, which is similar to the
traditional symbol for a digital
inverter that shows a buffer
followed by a bubble. In general,
the direction of the Schmitt trigger
(inverting or non-inverting) is not
necessarily clear from the symbol
because multiple conventions are
used, even with the same
manufacturer. There are several
factors leading to such
ambiguity,[nb 1] These
circumstances may warrant a closer
investigation of the documentation
for each particular Schmitt trigger.
Comparison between emitter- and collector-coupled circuit
The emitter-coupled version has the advantage that the input transistor is backward-biased when the input
voltage is quite below the high threshold; so, the transistor is surely cut-off. It was important when germanium
transistors were used for implementing the circuit and this advantage has determined its popularity. The input
base resistor can be omitted since the emitter resistor limits the current when the input base-emitter junction is
forward-biased.
The emitter-coupled Schmitt trigger has not low enough level at output logical zero and needs an additional
output shifting circuit. The collector-coupled trigger has extremely low (almost zero) output level at output
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logical zero.
Op-amp implementations
Schmitt triggers are commonly implemented using an operational amplifier
or the more dedicated comparator.[nb 2] An open-loop op-amp and
comparator may be considered as an analog-digital device having analog
inputs and a digital output that extracts the sign of the voltage difference
between its two inputs.[nb 3] The positive feedback is applied by adding a
part of the output voltage to the input voltage in series or parallel manner.
Due to the extremely high op-amp gain, the loop gain is also high enough
and provides the avalanche-like process.
Non-inverting Schmitt trigger
BJT bistable collector-base
coupled circuit can be converted to
a Schmitt trigger by connecting an
additional base resistor to some of
the bases
In this circuit, the two resistors R1 and R2 form a parallel voltage summer.
It adds a part of the output voltage to the input voltage
thus "helping" it during and after switching that occurs
when the resulting voltage is near the ground. This parallel
positive feedback creates the needed hysteresis that is
controlled by the proportion between the resistances of R1
and R2. The output of the parallel voltage summer is
single-ended (it produces voltage in respect to ground); so,
the circuit does not need an amplifier with a differential
input. Since conventional op-amps have a differential
input, the inverting input is grounded to make the
Schmitt trigger implemented by a non-inverting
reference point zero volts.
comparator
The output voltage always has the same sign as the op-amp
input voltage but it not always has the same sign as the circuit input voltage (the signs of the two input
voltages can differ). When the circuit input voltage is above the high threshold or below the low threshold, the
output voltage has the same sign as the circuit input voltage (the circuit is non-inverting). It acts like a
comparator that switches at a different point depending on whether the output of the comparator is high or
low. When the circuit input voltage is between the thresholds, the output voltage is undefined; it depends on
the last state (the circuit behaves as an elementary latch).
For instance, if the Schmitt trigger is currently in the high state, the output
will be at the positive power supply rail (+VS). The output voltage V+ of
the resistive summer can be found by applying the superposition theorem:
The comparator will switch when V+=0. Then
(the same result can be obtained by applying the current conservation
principle). So
Typical hysteresis curve
(Non-inverting) (which matches
the curve shown on a Schmitt
trigger symbol)
must drop below
to get the output to switch.
Once the comparator output has switched to −VS, the threshold becomes
to switch back to high. So this circuit creates a switching band
centered around zero, with trigger levels
(it can be shifted to the
left or the right by applying a bias voltage to the inverting input). The input voltage must rise above the top of
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the band, and then below the bottom of the band, for the output to switch on (plus) and then back off (minus).
If R1 is zero or R2 is infinity (i.e., an open circuit), the band collapses to zero width, and it behaves as a
standard comparator. The transfer characteristic is shown in the picture on the right. The value of the threshold
T is given by
and the maximum value of the output M is the power supply rail.
A unique property of circuits with parallel positive
feedback is the impact on the input source.
[citation needed]
In circuits with negative parallel
feedback (e.g., an inverting amplifier), the virtual
ground at the inverting input separates the input
source from the op-amp output. Here there is no
virtual ground, and the steady op-amp output
voltage is applied through R1 - R2 network to the
input source. The op-amp output passes an
opposite current through the input source (it injects
current into the source when the input voltage is
positive and it draws current from the source when
it is negative).
A practical Schmitt trigger configuration with precise
thresholds
A practical Schmitt trigger with precise thresholds
is shown in the figure on the right. The transfer
characteristic has exactly the same shape of the previous basic configuration, and the threshold values are the
same as well. On the other hand, in the previous case, the output voltage was depending on the power supply,
while now it is defined by the Zener diodes (which could also be replaced with a single double-anode Zener
diode). In this configuration, the output levels can be modified by appropriate choice of Zener diode, and these
levels are resistant to power supply fluctuations (i.e., they increase the PSRR of the comparator). The resistor
R3 is there to limit the current through the diodes, and the resistor R4 minimizes the input voltage offset caused
by the comparator's input leakage currents (see Limitations of real op-amps).
Inverting Schmitt trigger
In the inverting version, the attenuation and summation are
separated. The two resistors R1 and R2 act only as a
"pure" attenuator (voltage divider). The input loop acts as
a simple series voltage summer that adds a part of the
output voltage in series to the circuit input voltage. This
series positive feedback creates the needed hysteresis that
is controlled by the proportion between the resistances of
R1 and the whole resistance (R1 and R2). The effective
voltage applied to the op-amp input is floating; so, the
op-amp must have a differential input.
Schmitt trigger implemented by an inverting
comparator
The circuit is named inverting since the output voltage
always has an opposite sign to the input voltage when it is out of the hysteresis cycle (when the input voltage is
above the high threshold or below the low threshold). However, if the input voltage is within the hysteresis
cycle (between the high and low thresholds), the circuit can be inverting as well as non-inverting. The output
voltage is undefined; it depends on the last state and the circuit behaves as an elementary latch.
To compare the two versions, the circuit operation will be considered at the same conditions as above. If the
Schmitt trigger is currently in the high state, the output will be at the positive power supply rail (+VS). The
output voltage V+ of the voltage divider is:
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The comparator will switch when Vin = V+. So
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must exceed above this voltage to get the output to switch.
Once the comparator output has switched to −VS, the threshold becomes
to switch back to
high. So this circuit creates a switching band centered around zero, with trigger levels
(it can
be shifted to the left or the right by connecting R1 to bias voltage). The input voltage must rise above the top
of the band, and then below the bottom of the band, for the output to switch off (minus) and then back on
(plus). If R1 is zero (i.e., an short circuit) or R2 is infinity, the band collapses to zero width, and it behaves as a
standard comparator.
In contrast with the parallel version, this circuit does not impact on the input source since the source is
separated from the voltage divider output by the high op-amp input differential impedance.
Applications
Schmitt triggers are typically used in open loop configurations for noise immunity and closed loop
configurations to implement function generators.
Noise immunity
One application of a Schmitt trigger is to increase the noise immunity in a circuit with only a single input
threshold. With only one input threshold, a noisy input signal [nb 4] near that threshold could cause the output
to switch rapidly back and forth from noise alone. A noisy Schmitt Trigger input signal near one threshold can
cause only one switch in output value, after which it would have to move beyond the other threshold in order
to cause another switch.
For example, in Fairchild Semiconductor's QSE15x family of infrared photosensors,[5] an amplified infrared
photodiode generates an electric signal that switches frequently between its absolute lowest value and its
absolute highest value. This signal is then low-pass filtered to form a smooth signal that rises and falls
corresponding to the relative amount of time the switching signal is on and off. That filtered output passes to
the input of a Schmitt trigger. The net effect is that the output of the Schmitt trigger only passes from low to
high after a received infrared signal excites the photodiode for longer than some known delay, and once the
Schmitt trigger is high, it only moves low after the infrared signal ceases to excite the photodiode for longer
than a similar known delay. Whereas the photodiode is prone to spurious switching due to noise from the
environment, the delay added by the filter and Schmitt trigger ensures that the output only switches when there
is certainly an input stimulating the device.
As discussed in the example above, the Fairchild Semiconductor QSE15x family of photosensors use a Schmitt
trigger internally for noise immunity. Schmitt triggers are common in many switching circuits for similar
reasons (e.g., for switch debouncing).
List of IC including input Schmitt triggers
The following 7400 series devices include a Schmitt trigger on their input or on each of their inputs:
7413: Dual Schmitt trigger 4-input NAND Gate
7414: Hex Schmitt trigger Inverter
7418: Dual Schmitt trigger 4-input NAND Gate
7419: Hex Schmitt trigger Inverter
74121: Monostable Multivibrator with Schmitt Trigger Inputs
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74132: Quad 2-input NAND Schmitt Trigger
74221: Dual Monostable Multivibrator with Schmitt Trigger Input
74232: Quad NOR Schmitt Trigger
74310: Octal Buffer with Schmitt Trigger Inputs
74340: Octal Buffer with Schmitt Trigger Inputs and three-state inverted outputs
74341: Octal Buffer with Schmitt Trigger Inputs and three-state noninverted outputs
74344: Octal Buffer with Schmitt Trigger Inputs and three-state noninverted outputs
74(HC/HCT)7541 Octal Buffer with Schmitt Trigger Inputs and Three-State Noninverted Outputs
SN74LV8151 is a 10-bit universal Schmitt-trigger buffer with 3-state outputs
A number of 4000 series devices include a Schmitt trigger on inputs, for example:
4017: Decade Counter with Decoded Outputs
4020: 14-Stage Binary Ripple Counter
4022: Octal Counter with Decoded Outputs
4024: 7-Stage Binary Ripple Counter
4040: 12-Stage Binary Ripple Counter
4093: Quad 2-Input NAND
40106: Hex Inverter
14538: Dual Monostable Multivibrator
Dual Schmitt input configurable single-gate CMOS logic, AND, OR, XOR, NAND, NOR, XNOR
NC7SZ57 Fairchild
NC7SZ58 Fairchild
SN74LVC1G57 Texas Instruments
SN74LVC1G58 Texas Instruments
Use as an oscillator
Main article: Relaxation oscillator
A Schmitt trigger is a bistable multivibrator, and it
can be used to implement another type of
multivibrator, the relaxation oscillator. This is
achieved by connecting a single RC integrating
circuit between the output and the input of an
inverting Schmitt trigger. The output will be a
continuous square wave whose frequency depends
on the values of R and C, and the threshold points
of the Schmitt trigger. Since multiple Schmitt
trigger circuits can be provided by a single
integrated circuit (e.g. the 4000 series CMOS
device type 40106 contains 6 of them), a spare
section of the IC can be quickly pressed into
service as a simple and reliable oscillator with only
two external components.
Output and capacitor waveforms for comparator-based
relaxation oscillator
Here, a comparator-based Schmitt trigger is used in its inverting configuration. Additionally, slow negative
feedback is added with an integrating RC network. The result, which is shown on the right, is that the output
automatically oscillates from VSS to VDD as the capacitor charges from one Schmitt trigger threshold to the
other.
See also
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Hysteresis
Positive feedback
Operational amplifier applications
Bistable multivibrator circuit
Threshold detector with hysteresis
Comparator
Notes
1. ^ One factor contributing to ambiguity is that one simple
transistor-based realization of a Schmitt trigger is naturally
inverting, with a non-inverting Schmitt trigger sometimes
consisting of such an inverting implementation followed by an
inverter. An additional inverter may be added for buffering a
stand-alone inverting configuration. Consequently, inverting
A comparator-based implementation of a relaxation
configurations within an integrated circuit may be naturally
oscillator
inverting, while non-inverting configurations are implemented
with a single inverter, and stand-alone inverting configurations
may be implemented with two inverters. As a result, symbols that combine inverting bubbles and hysteresis
curves may be using the hysteresis curve to describe the entire device or the embedded Schmitt trigger only.
2. ^ Usually, negative feedback is used in op-amp circuits. Some operational amplifiers are designed to be used only
in negative-feedback configurations that enforce a negligible difference between the inverting and non-inverting
inputs. They incorporate input-protection circuitry that prevent the inverting and non-inverting inputs from
operating far away from each other. For example, clipper circuits made up of two general purpose diodes with
opposite bias in parallel [1] (http://www.analog.com/library/analogdialogue/archives/42-10/off_amps.html) or two
Zener diodes with opposite bias in series (i.e., a double-anode Zener diode) are sometimes used internally across
the two inputs of the operational amplifier. In these cases, the operational amplifiers will fail to function well as
comparators. Conversely, comparators are designed under the assumption that the input voltages can differ
significantly.
3. ^ When the non-inverting (+) input is at a higher voltage than the inverting (−) input, the comparator output
switches nearly to +VS, which is its high supply voltage. When the non-inverting (+) input is at a lower voltage
than the inverting (−) input, the comparator output switches nearly to -VS, which is its low supply voltage.
4. ^ Where the noise amplitude is assumed to be small compared to the change in Schmitt trigger threshold.
References
1. ^ Otto H. Schmitt, A Thermionic Trigger (http://dx.doi.org/10.1088/0950-7671/15/1/305), Journal of Scientific
Instruments 15 (January 1938): 24–26.
2. ^ a b August 2004 issue of the Pavek Museum of Broadcasting Newsletter - http://www.otto-schmitt.org
/Otto_Images/PavekOHSbio.pdf
3. ^ Debouncing switches with an SR latch (http://www.ee.nmt.edu/~elosery/fall_2008/ee231L/lab6.pdf)
4. ^ 7414 datasheet (http://www.datasheetcatalog.org/datasheets/400/334439_DS.pdf)
5. ^ Fairchild Semiconductor QSE15x photosensors: Product page (http://www.fairchildsemi.com/pf/QS
/QSE158.html), Datasheet (http://www.fairchildsemi.com/ds/QS/QSE158.pdf)
6 Strauss, Leonard (1970), Wave Generation and Shaping (2nd ed.), McGraw-Hill, ISBN 978-0-07-062161-9
External links
Calculator which determines the resistor values required for given thresholds (http://www.randomscience-tools.com/electronics/schmitt-trigger-calculator.htm)
Online Simulator of Op-amp Schmitt trigger (http://www.cirvirlab.com/simulation
/op_amp_schmitt_trigger_online.php) – Gives online simulation of Op-amp Schmitt trigger.
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